Site-Selective C-H Functionalization of Carbazoles

Carbazole alkaloids hold great potential in pharmaceutical and material sciences. However, the current approaches for C1 functionalization of carbazoles rely on the use of a pre-installed directing group, severely limiting their applicability and hindering their overall efficiency. Herein, we report for the first time the development of direct Pd-catalyzed C-H alkylation and acylation of carbazoles assisted by norbornene (NBE) as a transient directing mediator. Notably, the involvement of a six-membered palladacycle intermediate was suggested in this case, representing the first example of such intermediacy within the extensively studied Pd/norbornene reactions realm.

Construction of carbazole-based unnatural amino acid scaffolds via Pd(II)-catalyzed C(sp3)-H functionalization

We report the synthesis of carbazole-based unnatural α-amino acid and non-α-amino acid derivatives via a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided β-C(sp³)-H activation/functionalization method. Various N-phthaloyl, DL-, L- and D-carboxamides derived from their corresponding α-amino acids, non-α-amino acids and aliphatic carboxamides were subjected to the β-C(sp³)-H functionalization with 3-iodocarbazoles in the presence of a Pd(II) catalyst to afford the corresponding carbazole moiety installed unnatural amino acid derivatives and aliphatic carboxamides. Carbazole motif-containing racemic (DL) and enantiopure (L and D) amino acid derivatives including phenylalanine, norvaline, leucine, norleucine and 2-aminooctanoic acid with anti-stereochemistry and various non-α-amino acid derivatives including GABA have been synthesized. Removal of the 8-aminoquinoline directing group, deprotection of the phthalimide moiety and the preparation of carbazole amino acid derivatives containing free amino- and carboxylate groups are shown. The carbazole motif is prevalent in alkaloids and biologically active molecules and functional materials. Thus, this work on the synthesis of carbazole-based unnatural amino acid derivatives would enrich the libraries of unnatural amino acid derivatives and carbazoles.

Synthesis and optical characterization of bipod carbazole derivatives

In this study, some new biscarbazole derivatives were synthesized for the purpose of being used in OLED technologies and related areas. The following compounds: {1,2-bis(2-(3,6-diphenyl-9H-carbazole-9-yl) ethoxy)ethane (C-1), bis[2-(2-(3,6- diphenyl-9H-carbazole-9-yl) ethoxy)etyl]ether (C-2), bis[2-(2-(3,6-di(naphthalene-1-yl)-9H-carbazol-9-yl)ethoxy)etyl]ether (C-3) and bis [2-(2-(3,6-di(naphthalene-2-yl)-9H-carbazol-9-yl)ethoxy) ethyl]ether (C-4) were synthesized by Suzuki-Miyaura Cross Coupling reactions. The structural properties of the synthesized compounds were characterized by FT-IR, 1H-NMR, 13C-NMR, and LC-MS. The maximum product yields of 81.6% were obtained for C-4 biscarbazole derivatives. The optical properties were studied using UV-visible and temperature/excitation power density dependent photoluminescence (PL) techniques. The emissions were observed at green and yellow-red color spectral bands. By applying Gaussian fitting to the measured spectra, the superposition of the broad peaks was deconvoluted into two peaks. The origin of emissions was attributed to π- π* transition in aromatic compounds caused by intramolecular charge transfer from host carbazole to these compounds.

Synthesis of pyrazolino- and thiopyrimido- fused carbazoles from 2-ethylidene-3,4-dihydrocarbazol-1(2H)-ones

Annelated carbazoles namely, 3-methyl-1,2,3,3a,4,5,5a,10,10a,10b-decahydropyrazolino[4,5-a]carbazoles, 4a–e and 4-methyl-1,3,4,5,6,11-hexahydropyrimido[4,5-a]carbazole-2-thiones 5a–e were synthesised in excellent yields from 2-ethylidene-3,4-dihydrocarbazol-1(2H)-ones 2a–e, derived from the easily accessible 3,4-dihydrocarbazol-1(2H)-ones 1a–e.

PtCl4-catalyzed skeleton rearrangement–cyclization of tertiary indolyl-3-alkynols

A unique PtCl₄-catalyzed cyclization of tertiary indol-2-yl-3-alkynols provides an efficient access to potentially useful poly-substituted carbazoles.

One-pot relay catalysis: divergent synthesis of furo[3,4-b]indoles and cyclopenta[b]indoles from 3-(2-aminophenyl)-1,4-enynols

Described herein a divergent strategy for the synthesis of furo[3,4-b]indoles via a sequential Ag(i)/Bi(iii)/Pd(ii) catalysis and cyclopenta[b]indoles via a one-pot Ag(i)/Brønsted acid relay catalysis, starting from the easily accessible 3-(2 aminophenyl)-4-pentenyn-3-ols.

One-Pot Catalytic Asymmetric Synthesis of Tetrahydrocarbazoles

A one-pot asymmetric synthesis of 1,2,3,4-tetrahydrocarbazoles has been developed via an enantioselective [3 + 3] annulation of 2-alkynylindoles and donor-acceptor cyclopropanes. In the presence of chiral Lewis acids as catalysts, a series of optically active tetrahydrocarbazoles were furnished in high yields (63-87%) with good to excellent levels of enantioselectivity (up to 94% ee).

Gold(i)-catalysed cascade reactions in the synthesis of 2,3-fused indole derivatives

Unactivated alkenes and 1,3-unsubstituted indoles in gold(i)-catalysed hydroaminative/arylative cascade cyclizations.

Concise enantiospecific total synthesis of tubingensin A

We report the enantiospecific total synthesis of (+)-tubingensin A. Our synthesis features an aryne cyclization to efficiently introduce the vicinal quaternary stereocenters of the natural product and proceeds in only nine steps (longest linear sequence) from known compounds.

Palladium(ii)-catalysed total synthesis of naturally occurring pyrano[3,2-a]carbazole and pyrano[2,3-b]carbazole alkaloids

Seven naturally occurring pyranocarbazole alkaloids (pyrayafoline A–E,O-methylmurrayamine A andO-methylmahanine) have been synthesised using a palladium(ii)-catalysed cyclisation of a diarylamine to an orthogonally diprotected 2,7-dihydroxycarbazole as key step.

2-(6-Methyl-2,3,4,9-tetrahydro-1H-carbazol-1-ylidene)propanedinitrile

In the title compound, C₁₆H₁₃N₃, the cyclo­hexene ring adopts a sofa conformation. An intra­molecular N—H⋯N hydrogen bond generates an S(7) ring motif. In the crystal, the mol­ecules are linked by pairs of N—H⋯N inter­actions, forming centrosymmetric dimers with an R₂²(14) motif.

6-Bromo-2-(3-phenyl-allyl-idene)-2,3,4,9-tetra-hydro-1H-carbazol-1-one

Mol­ecules of the title compound, C₂₁H₁₆BrNO, are linked through pairs of N—H⋯O inter­molecular hydrogen bonds into centrosymmetric R₂²(10) dimers. One of the C atoms of the cyclohex-2-enone ring is disordered with refined occupancies of 0.61 (2) and 0.39 (2).

6-Bromo-2-[(E)-thio-phen-2-yl-methyl-idene]-2,3,4,9-tetra-hydro-1H-carbazol-1-one

In the title compound, C(17)H(12)BrNOS, the cyclo-hexene ring deviates only slightly from planarity (r.m.s. deviation for non-H atoms = 0.047 Å). In the crystal, the mol-ecules are linked into centro-symmetric R(2) (2)(10) dimers via pairs of N-H⋯O hydrogen bonds. The thio-phene ring is disordered over two positions rotated by 180° and with a site-occupation factor of 0.843 (4) for the major occupied site.

One-pot synthesis of new substituted 1,2,3,4-tetrahydrocarbazoles via Petasis reaction

The one-pot synthesis of a new substituted 1,2,3,4-tetrahydrocarbazoles has been described via Petasis reactions. These tetrahydrocarbazoles exhibits various medicinal importance and might be suitable for elaboration into larger peptides at carboxy termini. The scope and limitations of this method have been examined.